Photo‐Imprinting of the Helical Organization in Liquid‐Crystal Networks Using Achiral Monomers and Circularly Polarized Light

Abstract Control over molecular motion is facilitated in materials with highly ordered nanoscale structures. Here we report on the fabrication of cholesteric liquid‐crystal networks by circularly polarized light irradiation, without the need for chiral dopant or plasticizer. The polymer network is obtained by photopolymerization of a smectic achiral diacrylate mesogen consisting of an azobenzene core and discrete oligodimethylsiloxane tails. The synchronous helical photoalignment and photopolymerization originate from the cooperative movement of the mesogens ordered in well‐defined responsive structures, together with the flexibility of the oligodimethylsiloxane blocks. The resulting thin films show excellent thermal stability and light‐induced memory features with reversible responses. Additionally, we demonstrate the fabrication of photo‐patterned films of liquid‐crystal networks with opposite helical senses. These findings provide a new method to make light‐controllable chiroptical materials with exciting applications in optics and photonics.


Materials, instruments and protocols
All chemicals were purchased from commercial sources and used without further purification.
Dry solvents were obtained with an MBRAUN solvent purification system (MB-SPS). Ovendried glassware (120 °C) was used for all reactions carried out under argon atmosphere.
Reactions were followed by thin-layer chromatography (TLC) using 60-F254 silica gel plates from Merck and visualized by UV light at 254 nm.
Thin films preparation: Glass slides were cleaned by sonication with ethyl alcohol for 10 min.
These glass slides were then etched in a UV-ozone photoreactor (PR-100) for 20 min. To prepare a spin-cast thin film, a solution of the mixture in heptane (3 or 24 mg mL -1 ) was spincoated on the etched glass slides (5000 rpm, 40 s, 500 rpm acceleration). The film thicknesses were about 40 or 750 nm, respectively. To prepare the LCN, cells were made by gluing together two glass plates. Glass beads with a diameter of 6 µm were used to precisely control the gap of the cell (except for photo-patterning where beads of 2 µm were used). The cells were filled at the isotropic temperature of the mixture (around 70 °C) and polymerized in their LC phase with UV light source (EXFO Omincure-S2000) using a shorter wavelength (< 405 nm) cut filter for 20 min or with 405 nm or 450 nm LED (8 ± 1 mW cm -2 ) for 15-30 min. All thin films were stored in the dark and protected by aluminum foil.
Photo-alignment of spin-cast thin films: Photo-irradiation was carried out using a Thorlabs DC4104 advanced four-channel LED driver equipped with a DC4100-HUB. The LED driver 2 was employed under constant current operation. The light intensity was measured using a radiometer RM-12. To achieve LPL, an adjustable linear polarizer was used between the LED and the thin film. The linear polarizer was at 0° with respect to the s-direction of the Fresnel rhomb, with s the vector normal to the plane of incidence for the internal reflection in the rhomb.
CPL was achieved by adjusting the linear polarizer at 45° and 315° with respect to s. The handedness of the CPL was confirmed by a polarizer with the known CPL. The light intensity of the 405 nm and 450 nm LED with the linear polarizer and the Fresnel rhomb was 8 ± 1 mW cm -2 . A thin film was placed on a hot plate in a box filled with N2 gas and the film was kept at 25 °C, then the irradiation was started. After the irradiation, the film was removed from the hot plate and was stored in the dark with aluminum foil. LD and CD spectroscopy measurements were performed at 20 °C as soon as possible after the irradiation. 3 Polarized Optical Microscopy: POM was carried out using Nikon Eclipse Ci POL equipped with a Linkam LTS 420 heating stage.
Small Angle X-Ray Scattering: SAXS of thin films were performed on an instrument from Ganesha Lab. The flight tube and sample holder were all under vacuum in a single housing, with a GeniX-Cu ultra-low divergence X-ray generator. The source produces X-rays with a wavelength (λ) of 0.154 nm and a flux of 1 × 108 ph s -1 . Scattered X-rays were captured on a 2-dimensional Pilatus 300K detector with 487 × 619 pixel resolution. The sample-to-detector distance was 0.084 m (WAXS mode) or 0.48 m (MAXS mode), and the instrument was calibrated with diffraction patterns from silver behenate.